Stall Warning System

Introduction

Although the Grummans typically give a good tactile warning when a stall is imminent, the stall horn adds an additional aural warning. For those that have not noticed, the tactile warning is the shaking of the yoke due to wing wake impingement on the horizontal tail near and while in the stalled condition. In this article, the applicable regulations are presented, principles of operation, why preflight checks of the stall horn sometimes fail, adjustments, and general troubleshooting.

Applicable Regulations

14 CFR 23.207 states that

• (a) There must be a clear and distinctive stall warning
• (b) The stall warning may be furnished either through the inherent aerodynamic qualities of the airplane or by a device that will give clearly distinguishable indications under expected conditions of flight
• (c) ... the stall warning must begin at a speed exceeding the stalling speed by a margin of not less than 5 knots and must continue until the stall occurs.

Because of the tactile feedback due to the aerodynamic qualities of the stalls in Grummans, an audible stall warning may not be technically required. However, the Grummans were certificated with the stall horn.

Principles of Operation

Low Angle-of-Attack

Medium Angle-of-Attack

High Angle-of-Attack

The images to the right show the streamlines around an airfoil at progressively higher angles-of-attack. However, they do not show the separated streamlines typical of a stall. Notice that there is a streamline that terminates near the nose of the airfoil. The point of termination is called the stagnation point because this is where the flow reaches zero speed. More to the point, any streamlines above the stagnation streamline flow above the airfoil and any below the stagnation streamline flow below the airfoil.

Stall warning devices take advantage of this phenomenon. On the Grummans, a small tab sticks out of the leading edge. For normal angles-of-attack, the tab is positioned so that it is located below the stagnation streamline so that the normal flow forces the tab down and keeps the stall horn switch in the open position. As the angle-of-attack increases to stall, the stagnation point moves below the tab, and the streamlines act to lift the tab and close the switch thus activating the stall horn. Cessnas use a pressure activated switch rather than a tab but basically measure the location of the stagnation point in a similar way.

Preflight Checks

To check the condition of the stall horn on Grummans, the master switch must be on. For some models of Grummans, the stall horn is designed to have a several second delay ostensibly to avoid spurious activation of the horn near but above stall. Some have preflighted their stall horn and determined that it does not work without knowing about the several second delay.

Troubleshooting

The stall horn switch is located about midspan on the 4-seaters and is accessed via an access panel on the bottom of the wing. The switch on the 2-seaters is located near the wingtip and can be accessed by removing the fiberglass wingtip. The switch is held in place by an adjustment screw on the wing leading edge. The screw sits in a slot allowing for adjustment of the switch up and down. The horn is usually installed under the NACA inlet on the pilot's side on an L-shaped bracket.

The indicators that the stall warning system is not working correctly include

• Does not work at all
• Operates in flight but not on the ground
• Operates on the ground but not in flight
• Activates at relatively high speeds
• Activates in turbulence

Note: If you suspect a switch failure and want to remove it for testing, be sure to mark the position on the leading edge. Otherwise flight testing will be involved in getting it set in the correct position after troubleshooting.

1. Does not work at all. If the stall warning system does not work at all, first try testing it on the ground. Be sure to engage the switch with the master switch on for at least several seconds. If it works fine on the ground, the switch may need adjustment upwards. The switch may be positioned too low to be activated by the movement of the stagnation point near stall. If it does not work on the ground, the culprit could be a bad switch, bad horn, blown fuse.
2. Operates in flight but not on the ground. During preflight, are you holding the switch up for several seconds? Some of the stall warning systems operate on a delay to prevent nuisance activation. Also, some owners have reported that the culprit turned out to be low battery voltage that prevented the horn mechanism from activating. Once the engine was running and bus voltage was at 14V, the horn worked fine.
3. Operates on the ground but not in flight. The switch may be positioned too low. The stagnation point never moves underneath the tab even at stall conditions, so the tab will never be lifted to the on position. Loosen the adjustment screw and move the switch up in 1/4 inch or smaller increments each flight until it activates at least 5 kts above stall.
4. Activates at relatively high speeds. The switch may be positioned too high. For normal angles-of-attack, the stagnation point is moving under the switch and activating it. Loosen the adjustment screw and move the switch downward in 1/4 inch or smaller increments each flight until it activates at least 5 kts above stall.
5. Activates in turbulence. The component that produces the delay may have failed. The delay is meant to prevent nuisance activation of the stall horn by relatively short duration angle-of-attack excursions cause by turbulence.

If switch failure is suspected, first mark the position of the switch on the leading edge. Remove the adjustment screw, disconnect the wiring, and test the switch using a multimeter. Some owners have reported blowing dust out/off of the switch bringing it back to life. Use low pressure compressed air such as the stuff in cans used for cleaning computer components. Spraying electronic contact cleaner into the switch may remove oxidation from the internal contacts. Also check for oxidation and corrosion on the external terminals.

Horn failure can be tested by applying 12V directly to the horn terminals. One owner reported ...

This kind of horn has been used as an inexpensive alarm sounding device for alot more years that than I can remember and basically it is a smaller (quieter) version of the way most auto horns were fabricated for years... and unless the coil inside is fried (open) you can normally free up the stuck reed by exercising it with your finger... just press the reed up and down so that you hear a clicking sound this will indicate you have free'd up the reed and the bimetal strip should react to the em [electromagnetic] coil when it is energized. I gave my assembly a quick shot of lps silicon spray, wiped it off, retested with the battery and "good to go", master on, trip the stall switch, just like new.